The present invention relates to well treatment compositions and methods utilizing nano-particles and, more particularly, to well cement compositions and methods utilizing particulate nano-silica.
In general, well treatments include a wide variety of methods that may be performed in oil, gas, geothermal and/or water wells, such as drilling, completion and workover methods. The drilling, completion and workover methods may include, but are not limited to, drilling, fracturing, acidizing, logging, cementing, gravel packing, perforating and conformance methods. Many of these well treatments are designed to enhance and/or facilitate the recovery of desirable fluids from a subterranean well.
In cementing methods, such as well construction and remedial cementing, well cement compositions are commonly utilized. For example, in subterranean well construction, a pipe string (e.g., casing and liners) may be run into a well bore and cemented in place using a cement composition. The process of cementing the pipe string in place is commonly referred to as “primary cementing.” In a typical primary cementing method, a cement composition may be pumped into an annulus between the walls of the well bore and the exterior surface of the pipe string disposed therein. The cement composition sets in the annular space, thereby forming an annular sheath of hardened, substantially impermeable cement that supports and positions the pipe string in the well bore and bonds the exterior surface of the pipe string to the subterranean formation. Among other things, the annular sheath of set cement surrounding the pipe string functions to prevent the migration of fluids in the annulus, as well as protecting the pipe string from corrosion. Cement compositions also may be used in remedial cementing methods, such as squeeze cementing and the placement of cement plugs.
In operation, the annular sheath of cement formed between the well bore and the pipe string often suffers structural failure due to pipe movements which cause shear stresses to be exerted on the set cement. Such stress conditions are commonly the result of relatively high fluid pressures and/or temperatures inside the cemented pipe string during testing, perforating, fluid injection or fluid production. For example, such stress may occur in wells subjected to steam recovery or production of hot formation fluids from high-temperature formations. The high-internal pipe pressure and/or temperature can result in the expansion of the pipe string, both radially and longitudinally, which places stresses on the cement sheath causing the cement bond between the exterior surfaces of the pipe or the well bore walls, or both, to fail and thus allow leakage of formation fluids and so forth. Accordingly, it may be desirable for the cement composition utilized for cementing pipe strings in the well bores to develop high strength after setting and to have sufficient resiliency (e.g., elasticity and ductility) to resist loss of the cement bond between the exterior surfaces of the pipe or the well bore walls, or both. Also, it may be desirable for the cement composition to be able to resist cracking and/or shattering that may result from other forces on the cement sheath. For example, it may be desirable for the cement sheath to include structural characteristics that protect its structural integrity from forces associated with formation shifting, overburden pressure, subsidence, tectonic creep, pipe movements, impacts and shocks subsequently generated by drilling and other well operations.
In addition to including components that improve mechanical properties of the cement, in a number of cementing methods, it may also be desirable to include one or more set accelerators in the well cement compositions to counteract certain constituents and/or environmental characteristics that excessively slow set times. For example, among other things, low temperatures and cement additives (e.g., fluid loss control additives and dispersants) can cause or contribute to an excessive set time for a cement composition. Accordingly, in certain situations, it may be desirable to reduce the set time by including a set accelerator in the cement composition. That is, the set accelerator may be included in a cement composition to counteract components of the cement composition or conditions surrounding the cement composition that are causing an excessive set time.